Reaching similar goals by different means – Differences in life-history strategies of clonal and non-clonal plants

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Highlights

  • Clonal growth determines plant’s position in life-history trait space.

  • Axes of life-history space are correlated with indicator values for disturbance.

  • Clonal species are shifted more towards the slow-end of slow-fast continuum.

  • Clonality serves more for rapid boom of populations than for mitigating busts.

Abstract

Clonality is a largely underexplored plant life-history trait with possibly profound effects on plant demography. Clonal growth constitutes an alternative reproductive pathway, which should provide clonal species with an advantage over non-clonal ones under disturbance regimes unfavourable to regeneration from seeds.

We investigated how clonal and non-clonal species differ in their life histories (other than clonality) and how this relates to disturbance regimes where the studied species occur. Further, we focused on the contribution of clonality to fluctuations in the populations of species and the importance of clonality for the life cycle of a species in relation to its other life-history characteristics. We achieved this through phylogenetically informed analyses of the matrix population models available from the COMPADRE database coupled with information on species clonality from the CLO-PLA database.

The phylogenetic principal component analysis revealed that plant life-history characteristics could be aligned along two gradients. The gradient of generation time and individual turnover in populations was more important and corresponded to the frequency of habitat disturbance. Clonal species on average had populations with lower overall rates of individual turnover and disturbance frequencies. The second gradient was correlated with disturbance severity and plant ability to regenerate after the loss of biomass. The importance of clonal growth for the life cycle of a clonal species increased with more severe disturbance events. The fluctuation of population growth rates depended on the life-history characteristics of a species but not on clonality. The net effect of clonal growth on the fluctuations of the populations of a species was positive.

In general, clonality seems to provide an important alternative for adjusting plant life history to the disturbance regime and other site conditions allowing a plant to circumvent its morphological or developmental constraints. Clonal growth turned out to be mainly a mechanism that enables population expansion under favourable conditions rather than a mechanism that buffers the effects of adverse conditions.

Introduction

The seminal work by Harper (1977) has drawn the focus of plant ecologists to the study of the entire plant life cycle which founded plant demography in its current sense. The accumulating body of studies (Salguero-Gómez et al., 2015) later enabled asking comparative questions on plant life histories using matrix population models (hereafter MPMs; Caswell, 2001; Sarukhán and Gadgil, 1974). Existing comparative analyses jointly identified the pace of individual turnover in populations as the main axis of variation in plant life histories, although they analysed often very different MPM characteristics (Franco and Silvertown, 2004; García et al., 2008; Salguero-Gómez et al., 2016; Silvertown et al., 1993). This principal axis of plant life-history variation has often been called the “fast-slow continuum” (see e.g. Salguero-Gómez et al., 2016; a term we adhere to throughout the article). The same study also discovered a secondary axis of life-history variation termed the “reproductive strategy” continuum, ranging from iteroparous plants to semelparous plants for which retrogression was very important.

The existing comparative analyses of plant life history spectra have in common the focus on plants’ ability to grow, persist, and reproduce generatively and do not pay much attention to clonal growth (but see Salguero-Gómez, 2018). However, clonally produced rooted units (new ramets of a genet sensu Harper, 1977) are capable of becoming physiologically independent of the mother ramet, i.e. clonal growth can produce new individuals from the demographic point of view (see e.g. Piqueras and Klimeš, 1998). Including clonal growth explicitly into comparative demographic analyses as a means of the production of new individuals thus has the potential to change the positions of clonal species on the fast-slow continuum (especially if we consider the trade-off between sexual and clonal reproduction; Benson and Hartnett, 2006; Herben et al., 2015). To the extent of our knowledge, this role of clonal growth in shaping plant life histories has not been examined, possibly due to the underrepresentation of (at least temperate) clonal plant species in the existing databases of MPMs (Janovský et al., 2017).

Clonal growth has been predicted to be beneficial for species’ population dynamics mainly under conditions that limit successful generative reproduction (Aarssen, 2008) and/or under conditions favouring regeneration over seeding, e.g. under intermediate disturbance frequencies (Bellingham and Sparrow, 2000). Such predictions should translate into a shift of a clonal species towards the slow end of the fast-slow continuum, which seems to be corroborated by a higher proportion of clonal species in habitats with low to intermediate disturbance frequencies and harsh abiotic conditions (Herben et al., 2018; Klimeš et al., 1997). The intensity of clonal growth can also vary over time (Tolvanen et al., 2001). Clonal growth can either boost population growth in favourable conditions or help to buffer unfavourable periods with high mortality and little generative reproduction.

We focused on four particular questions concerning the role of clonal growth in demography: 1) Are clonal species shifted towards the “slow end” of the fast-slow continuum (prediction of Aarssen, 2008)? 2) Is there a relationship between plant life-history characteristic space (ordination space of plant life history strategies) and disturbance frequency and severity? And do clonal species prefer intermediate disturbance levels (prediction of Bellingham and Sparrow, 2000)? 3) How does clonality and other species’ life history characteristics relate to variation of its population growth rates? 4) Does the importance of clonality for the life cycle of a species and the contribution of clonality to population fluctuations depend on the position of a species in the life-history characteristic space? We answered these questions with phylogenetically informed analyses of the MPMs available from the COMPADRE database (Salguero-Gómez et al., 2015) coupled with information on species’ clonality contained in the CLO-PLA database (Klimešová et al., 2017). We limited our analyses to clonal and non-clonal perennial herbs due to the lack of data on the demography of clonal trees/shrubs and due to the impossibility to capture the clonality of annuals similarly to that of perennials by standard MPMs.

Section snippets

Dataset compilation

MPMs were originally compiled from the COMPADRE Plant Matrix Database, version 4.0.1. We selected species meeting the following criteria. 1) Information on whether a species was clonal (clonal growth possible yes/no) was available in the CLO-PLA database, version 3.4. 2) For clonal species, clonal growth transitions were reported separately from other transitions (i.e. in a submatrix of clonal transitions). 3) The transition interval was one year (or could be recalculated to that, see Appendix

Life-history characteristic space

The first two axes of the phylogenetic PCA explained together 72.3% of the variation in plant life histories. The phylPCA1 corresponded to the fast-slow continuum detected already in earlier studies (Fig. 1). Fecundity and generation time had the greatest loadings on the first component followed by survival and progression (Table 1). PhylPCA2 established a gradient from species with a high proportion of flowering individuals in SSD and an elasticity of λ to retrogression close to zero

Discussion

The phylogenetic PCA identified two key axes of plant life-history characteristics – the first was the “fast-slow” continuum corresponding to the gradient of disturbance frequency, while the second axis corresponded to a gradient of disturbance severity. Clonal species did not occupy the extremes of the fast-slow continuum and their centroid was shifted more towards the “slow end” (Question 1) in line with the prediction of Aarssen (2008). The fast-slow continuum was correlated with the

Author contributions

ZJ, TH conceived the research. ZJ gathered the data from databases and prepared them for analyses. ZJ conducted the analyses. ZJ and TH interpreted the results. ZJ wrote the manuscript, to which TH made important contributions by editing.

Acknowledgements

Our cordial thanks are due to Zuzana Münzbergová who gave us access to additional demographic data on several of the clonal species. We are also deeply indebted to Jitka Klimešová for inspiring and discussing our research and to two anonymous reviewers for providing many valuable comments to the manuscript. The work has been supported by the Czech Science Foundation grants no. GB14-36079G (ZJ and TH) and no. GA19-13231S (TH). ZJ has been also supported by Charles University Research Centre

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